1. Field of the Invention
The present invention relates to a method and apparatus for discharging high voltage and more particularly to an apparatus for discharging residual high voltage present on pressurized liquid containers utilized in elecrostatic spray painting equipment which applies water reducible coatings. The invention further relates to a method and apparatus for isolating high voltage electrostatic
2. Background of the Art.
Many commercial applications call for the application of smooth thin coats of water reducible paint to very large surfaces. Spray painting is the technique most often used to accomplish this in time and cost effective manner with a minimum of material. However, high pressure paint sprays or atomized paint streams may fail to deposit a large percentage of the paint on the surface being painted. Air turbulence, ventilation currents, and surface recoil interactions often disburse a fair amount of the pain into the air where it is distributed onto other surfaces or remains suspended for an extended period. This results in a decreased transfer efficiency for the paint material and also presents safety problems.
Many primers, thinners, and paint materials create biological health hazards when distributed in significant quantities in the air. Water soluble primers and the like can be readily absorbed by human physiological systems and cause a variety of health risks. Various Federal and State safety regulations often require that a minimal amount of sprayed paint or particulate matter adhere to the surfaces. being painted to prevent exposure to air borne substances. Therefore, to improve the cost effectiveness and health risks, electrostatic painting processes are typically used to assure that a minimum percentage of the pain particles, on the order of 6l% or more, reach and adhere to the surface being painted.
In an electrostatic painting process, the paint is typically forced through a charged grid as part of the pressurized spraying process. The grid is typically charged to a voltage in excess of 75-100,000 volts at a negative or positive polarity and the working piece being painted is fixed at a ground potential at the opposite polarity. This means that the paint particles are either negatively or positively charged and are strongly attracted to the opposing charge of the painted surface which causes the particles to adhere strongly and rapidly to that surface. This technique reduces the loss of paint into the air to acceptable minimums.
Unfortunately, electrostatic painting also has associated safety hazards in the areas of electrical shock and explosion. The pressurized containers or pots holding the paint develops an accrued electrostatic, surface, charge on the order of 75-100,000 volts and is capable of delivering a considerably high current discharge to anyone coming in contact with the container. These containers have to be placed on electrically insulated stands for operation and are often surrounded by safety barriers to prevent contact by painting personnel. In addition, a grounding strap, probe, or similar discharge element is provided to discharge any residual electrostatic charge remaining on the container at the end of its use.
Generally a control unit associated with the electrostatic charging assembly is designed to drain off residual charge once the unit is no longer functioning. However, it is quite common that existing control units are not capable of handling all of the charge present on the pressurized container and the use of a safety grounding probe does in fact discharge a substantial amount of voltage. When any discharge occurs to through a grounding probe, it creates a discharge arc through the air surrounding the portion of the probe closest to the pressurized container. This gives rise to the potential for initiating an explosion.
Although an electrostatic process is more efficient at transferring material to a painted surface, there is still a significant percentage of material present in the air adjacent to the pressurized container. In addition, painting is often carried on in a paint shop or work area where other solvents and materials are being deposited into the air. Since many paints, primers, and solvents being sprayed are potentially very volatile and explosive, an electrical discharge could prove disastrous.
Current safety discharge mechanisms and discharge control systems do not guarantee a full and complete discharge of residual charge from paint containers nor do they prevent the presence of discharge arcs in the air surrounding pressurized containers. What is needed, then, is a method or apparatus which safely guarantees the discharge of any residual electrostatic charge on such pressurized containers. It is also desirable that any such apparatus comprise a mechanically simple and cost efficient device which can be readily adapted for use on a variety of electrostatic painting equipment.